How cells determine when and where to divide remains one of the great mysteries of modern biology. Spatially, division is tightly regulated to ensure the accurate positioning of septa. Temporally, division is coordinated with DNA replication and chromosome segregation. From bacteria to yeast to humans, cell division is initiated by the formation of a ring of a cytoskeletal protein at the nascent division site. This ring establishes the location of the division septum and serves as a framework for assembly of the division apparatus. In bacteria this ring is composed of the essential tubulin-like GTPase FtsZ. This proposal focuses on the regulatory networks that govern FtsZ ring formation in the soil bacterium B. subtilis. The factors that establish the division site and couple FtsZ ring formation to the cell cycle remain unknown. Comprehending the spatial and temporal regulation of bacterial division thus requires the identification of the cellular and molecular mechanisms that stimulate FtsZ ring formation at midcell in response to cell cycle cues and inhibit FtsZ ring formation at all other sites. EzrA, a factor that helps restrict FtsZ ring formation to midcell, was identified through classical genetic screens. Preliminary biochemical data suggest that EzrA interacts directly with FtsZ to inhibit ring formation by destabilization of FtsZ polymers. This proposal has three major goals. One, to characterize the molecular mechanism by which EzrA prevents ectopic FtsZ ring formation. Two, to clone and to characterize the gene identified by wee2, a mutation that uncouples cell division from growth, leading to the formation of small cells, many less than 25% the size of wild type B. subtilis. Three, to extend genetic screens to identify additional factors that (i) promote FtsZ ring formation at midcell, (ii) couple FtsZ ring formation to the cell cycle, and (iii) inhibit FtsZ ring formation at inappropriate sites. As essential components of the bacterial cell division machinery, FtsZ and the factors governing its activity hold promise as potential targets for the development of new antibiotics. Furthermore, this work should illuminate not only bacterial cell division, but also aspects of cytokinesis fundamental to all organisms. Understanding the molecular mechanisms that normally control cell division will help identify why they fail during oncogenesis, leading to the aberrant divisions and rapid proliferation characteristic of cancer cells.